DevTools: merge TracingTimelineModel into TimelineModel

Now that there is only one implementation it doesn't make sense to separate interface from implementation of TimelineModel.

BUG=428700

Review URL: https://codereview.chromium.org/715803002

git-svn-id: svn://svn.chromium.org/blink/trunk@185130 bbb929c8-8fbe-4397-9dbb-9b2b20218538